The invention relates to a technique for the manufacture of a gear rack for rack-and-pinion steering gears for automobiles, and to a gear rack manufactured by such a technique.
Such a technique is known, for example, from DE 32 02 254 C2. With this technique, a cylindrical blank between an upper and a bottom tool is formed into a gear rack by means of tumble-forging. With this technique, as with all forging and pressing techniques, a burr is created, which in the known technique is specifically used as an extension of the cross-section of the gear rack. This increases gear rack resistance. However, it also increases installation space.
With regard to rack-and-pinion steering gears for automobiles, smaller installation space requirements and savings in weight are repeatedly requested.
The object of the invention is to present a gear rack and a production method for such a gear rack which can meet the requests for smaller installation space and for savings in weight.
This object is solved by the technique presented hereafter, and by the gear rack described hereafter.
The technique for manufacturing such a gear rack begins with the selection of a gear rack blank, which at least in the toothed area has a form deviating from a circular cylinder. Size and shape of the cross-section of the blank are selected so that the contour of the finished rack in the toothed area does not project beyond the envelope given by the dimensions of neighboring parts of the rack-and-pinion steering gear. Then the gear rack is formed from the blank by means of a tumble-forging technique. After tumble-forging, the gear rack in its toothed area outside the toothed part has a convex or concave longitudinal profile, which during tumble-forging serves to support the tumbling torques.
Purposeful and expedient further developments of the technique are also described in hereafter. During tumble-forging, the longitudinal profile supports the tumbling torques, and in the finished rack it improves function by means of precise guidance and support on a pressure piece of the rack-and-pinion steering gear. The longitudinal profile can be provided on the blank during tumble-forging. This allows use of a blank with a relatively simple outer contour. But if it is possible to provide this longitudinal profile on the blank prior to tumble-forging, e.g., by stamping, pulling, or rolling, then tumble-forging can be limited to the forming of the blank in the toothed area of the gear rack. The specification that the contour of the finished gear rack in the toothed area should not project beyond a certain envelope can be met where the blank in the toothed area of the gear rack has at least flattening areas converging on each other, or two essentially symmetrically arranged X-section necks. For further weight savings, the blankxe2x80x94at least in the toothed area of the gear rackxe2x80x94can have at least one cavity extending in a longitudinal direction of the gear rack.
Since with the invented technique a lower degree of forming is achieved, a low-ductile material can be used. This allows additional reduction of gear rack costs.
A gear rack manufactured by this technique has a toothed area and at least one bearing and guiding area, just like other gear racks. But the gear rack manufactured by the invented technique has a contour, which in the toothed area does not project beyond an envelope given by the dimensions of neighboring parts of the rack-and-pinion steering gear. The envelope diameter of a rack-and-pinion power steering gear is given, for example, by the inner diameter of a sealing in the bearing and guiding area of the gear rack. The convex or concave longitudinal profile in the toothed area of the gear rack is ideally formed by a longitudinal groove located on the outer circumferential surface of the gear rack on the side opposite the teeth.